Overhead doors, as encountered in both commercial and residential settings represent a space-saving way of providing ingress into buildings. Overhead door is defined herein to include vertically rolling steel, sectional, countershutters, grilles, doors, as well as single section closures which retract and engage via a rolling track. Since the weight of such doors must be wholly displaced vertically, a counterweight is usually provided to lessen the work of operation. Counterbalancing mechanisms are typically employed in this role due to their compactness and high mechanical energy storage densities. Using dead weight as a counterbalance is often inefficient in that it occupies considerable space and has an appreciable travel distance.
The disadvantage of storing mechanical power within a mechanism is the loss of elastic compression with wear, resulting in a loss of counterbalancing ability. In the extreme case, the mechanism fails catastrophically leaving the unbalanced door load to be driven by a now overtaxed motor. The drive motor for overhead doors typically are connected to the rolling door spindle or track via gearing and a chain or belt drive. The diminished counterbalancing ability of a fatigued or broken mechanism burdens the chain drive and gear teeth with a greater load density per unit of weight-bearing area. This situation not only taxes the motor and all mechanical parts, but also leads to chain drive slippage, or failure and the possibility of the door free-falling in the course of operation.
Various methods for preventing the accidental unrolling of an overhead have been developed. These methods have generally been responsive to an excessive and unsafe falling speed of the door. Such excessive falling speeds induce a mechanical brake mechanisms to engage thereby limiting the descent of the door. The braking mechanisms have included bearing and inertial type systems. U.S. Pat. No. 4,848,522 is representative of such safety devices. Regardless of the specifics, currently used methods of preventing the accidental unrolling of overhead doors only respond to a catastrophic failure of the door lifting assembly whereby the door is in free fall. The engagement of this type of device, a stop lock bearing or inertial brake, locks the door from moving in either direction and, requires service when tripped to relieve the tension on the device for re-setting. The installation costs and service requirements are major factors in limiting the utility of these devices. There is no indication with these devices that such an event is imminent and upon preventing such a free fall, the door inevitably seizes at the point in its travel when the door dropping speed became excessive. Furthermore, should such a safety mechanism malfunction, harm is a genuine possibility.
It is an object of the invention to provide a warning that an uncontrolled door descent is imminent, thereby allowing for interdiction prior to door free fall.